In the CDMA system, on the transmission side, radio channels are each subjected to code modulation using a spreading code specific to each radio channel, then multiplexed, and transmitted, and the receiving side demultiplexes each channel by decoding operation called despreading using a code specific to that radio channel. To this end, the receiving side should perform despreading using the identical spreading code and on the same timing as the transmission side. Generally, in a mobile communication system, a different spreading code is used for each call connection, and accordingly, it is necessary to previously inform the receiving side of a spreading code to use.
Thus, the CDMA mobile communication system sets a broadcast channel called a perch channel, in addition to a channel through which user data for communication is transmitted, and uses this perch channel for transmitting code information required for despreading. This perch channel and the other channels (a control channel, a traffic channel, etc.) are subjected to code modulation using respective spreading codes orthogonal to one another to separate those channels. Those spreading codes for channel separation are characteristic to the system and therefore known to mobile stations (MS). Here, the perch channel is a broadcast channel for transmitting common control information to a plurality of mobile stations.
On the other hand, in the CDMA system, it is required that a mobile station distinguishes each base station (BS). Each base station spreads the perch channel with a spreading code unique to that base station, to transmit the perch channel. Namely the perch channel is subject to spreading twice, once with the spreading code for the channel separation, and once with a spreading code for distinguishing a base station. Differently from the spread code for channel separation, this spreading code for distinguishing a base station is unknown to a mobile station. Accordingly, a mobile station must try and find whether a spreading code for distinguishing a base station coincides with one of a plurality of candidates, one by one.
FIG. 9 is a diagram for explaining a perch channel signal system according to the above-mentioned conventional technique, and now it will be described furthermore. In FIG. 9, the symbol A indicates one slot of the perch channel, and the symbol B indicates one slot of the traffic channel. The perch channel transmits codes used by a mobile station for despreading, and the traffic channel transmits a user traffic such as a call traffic and data traffic. Each slot A of the perch channel is subjected to code modulation twice, once with a short code C1 unique to the system and common to every base station within the system, and once with a long code D1 unique to a particular bane station.
The perch channel and a group of essential traffic channels (a plurality of traffic channels exist correspondingly to the number of mobile stations under communication) transmitted by the base station at the same time are each spread with spreading codes orthogonal to one another to avoid mutual interference. On the other hand, the system structure becomes simple when the code D1 unique to a particular base station is common for all the traffic channels emitted from that particular base station. Thus, each slot B of the traffic channels other than the perch channel emitted from the same base station is subjected to spreading twice, once with a short code CN that is orthogonal to and different from the short code C1 that is unique to the system and used for the perch channel, and once with the long code D1 that is same as the perch channel and unique to a particular base station. By using CN that is orthogonal to C1 in terms of a code, it is possible to ensure orthogonality between the perch channel and traffic channels.
However, in the above-described example, since the code D1 unique to a base station is not previously informed to a mobile station, a mobile station must try all the codes that can be considered in principle, to search out the correct code. In this method, when the number of kinds of those codes is enormous, or when the code length of those codes is extremely long, much time is required for searching out the correct code.
Thus, it is possible to consider a method where the codes assigned to the base stations are classified into some groups in advance, and classification information is transmitted in advance. In this case, there is known a method of shortening the search time, in which a mobile station first detects the group to which the required code belongs, to narrow the range of search objects, and then, searches the codes within that group.
Further, in order to perform despreading with a long code D1 unique to a base station, it is necessary to know the time (timing of the code) at which despreading is started using that code. However, in the example shown in FIG. 9, it is impossible to know the start time before identifying the code D1, which is a problem to solve.
FIG. 10 is a diagram for explaining a perch channel signal system according to another conventional technique that can solve the above-mentioned problem, and it will be described in the following.
The example shown in FIG. 10 uses two kinds of perch channels, i.e., first and second perch channels, as the perch channel. Each slot of the first perch channel comprises an area A1 and an area C. Similarly to the case of FIG. 9, the area A1 is subjected to code modulation twice, once with the short code C1 unique to the system and common to every base station within the system and once with the long code D1 unique to a particular base station. And, the area C is subjected to spreading with a spreading code different from those codes. At this area C, spreading with the long spreading code D1 and the spreading with the short spreading code C1 unique to the system are stopped, and instead, this area C is spread with a short spreading code O2.
This spreading code O2 is used for clarifying the send timing of the perch channel, and a mobile station can realize slot synchronization of the perch channel, by receiving the area C. Namely, the mobile station can know the time at which the long spreading code D1 started. Further, a code predetermined by the system can be used as the spreading code O2, similarly to the spreading code C1, and, in order for a mobile station to easily establish slot synchronization, a special code having a small correlation is used.
Further, in the second perch channel, an area D is set at a time position that is synchronous with the area C of the first perch channel. This area D is spread with a short spreading code O3 that indicates a class (group) to which the long spreading code D1 unique to the base station belongs. Here, the codes O2 and O3 have orthogonal relation with each other, and therefore, can be separated even if they overlap in time base with each other. An interval between a certain area D and the next area D, namely, a part that is synchronous with an area A1 of the first perch channel, is made empty.
As the conventional technique concerning a mobile communication method using the above-described CDMA multiplexing technique, is known the technique described in HIGUCHI Ken-ichi, SAWAHASHI Mamoru and ADACHI Fumiyuki, “Method of high speed cell search using a long code mask in a DS-CDMA cellular system with asynchronous base stations”, Shingakugiho, TECHNICAL REPORT OF IEICE DSP96–116, SAT96–111, RCS96–122 (1997–01), for example.
In the above-described conventional technique shown in FIG. 10, the long spreading code D1 is not used for spreading in the areas C and D of the first and second perch channels, and, the short spreading codes O2 and O3 are not necessarily orthogonal to the spreading code C1. Accordingly, the parts of the areas C and D are not orthogonal to communication channels other than the perch channels. Accordingly, it is possible that, in the above-described conventional technique, signals in the parts of the areas C and D of the first and second perch channel interferes with the communication channels, causing interference similar to white noise.
Owing to the mentioned interference, partial noise is mixed onto a signal on the essential traffic channel. Generally, however, by sufficient interleaving in the traffic channel, a partial error is not made and a severe problem does not occur as a result of averaging.
However, a general traffic channel is multiplexed with call control information for ring trip, handover, call termination, and the like. Thus, when the above-mentioned area C or D overlaps in time base with an area of the call control information, severe interference is given to the call control information.
FIG. 11 is a diagram for explaining that the areas C and D give interference to the call control information on the traffic channel, and it will be described in the following.
In FIG. 11, the first and second perch channels have the same structures as ones described referring to FIG. 10. The traffic channel for an essential user signal has areas F for transmitting the call control information used for ring trip, handover, call termination, and the like, in addition to areas E for transmitting essential signal information. Here, the areas F–E are spread with the spreading codes CN and D1 as described above. The call control information requires a lower bit rate than the essential signal information, and accordingly, as shown in FIG. 11, the control information is time division multiplexed as the area F having shorter duration with the area E for transmitting the signal information. The length of one slot consisting of the area E for the traffic channel and the area F is same as the length of one slot consisting of the area A and the area C of the first perch channel, in terms of time duration.
Further, for the sake of convenience of description, FIG. 11 shows the case where the areas C and D of the first and second perch channels and the areas F of the traffic channel have completely synchronized timing. However, a different slot offset is set for a traffic channel of each mobile station, and it is possible that the area F partly overlaps with the areas C and D.
Thus, in the conventional technique shown in FIG. 10, when the areas C and D of the perch channel and the area F for transmitting the call control information overlap with each other, interference with the call control information is generated, as shown in FIG. 11. Generally, in the case of user information such as voice or data, generation of an occasional error does not invite fatal degradation of the service. On the other hand, it is an important problem that an error in the call control information invites unstableness of call connection and considerable degradation of the service. In particular, the area F for transmitting the call control information has shorter duration in comparison with the areas E and A1, the area F may entirely overlap with the areas C and D. Thus, when almost all the area F overlaps with the areas C and D, improvement in error can not be expected by interleaving the area F only.